Occurrence and Environmental Implications of Calcareous Nannofossils in Surface Sediments of the Western Gulf of Guinea: off Lagos Coast, South-western Nigeria

The application of calcareous nannofossils in oil exploration, and environmental studies is playing an increasingly important role all over the world. Most recent researches on this subject area in Nigeria are confined/restricted to the oil producing Niger Delta leaving most other areas untouched. This however limits nannofossils’ application in biostratigraphic and paleoenvironmental interpretation of non oil producing areas within the country. Before now, nannofossils occurrence and application have not been documented in the surface sediments of Nigeria. Consequently, calcareous nannofossils have been investigated in thirteen (13) surface sediments samples of the littoral area from the continental shelf of western Gulf of Guinea, off Lagos coast. Primarily, this research was aimed at determining the occurrence (and or non occurrence), and relative abundance of nannofossils in surface sediments of the area with which the age of the sediments, paleoenvironment and paleoecological condition(s) could be determined.The samples were collected with the aid of grab sampler, allowed to dry and subjected to detailed lithologic description. Furthermore, the samples were subjected to nannofossils analysis by standard method of preparation. Both qualitative and quantitative analysis was done using the high power Olympus Transmitted Light Microscope (TLM). The occurrence of nannofossils has been established in the study area (littoral area of the western Gulf of Guinea). Most of the sediments are poorly consolidated, fine to coarse grained sand with little amount of clay ant silt fractions that yielded Pliocene to Recent nannofossils (mainly Pleistocene). A total population of 1132 belonging to five different genera (Calcidiscus, Ceratolithus, Gephyrocapsa, Helicosphera and Reticulofenestra) and seven species were found in the studied samples with Gephyrocapsa caribbeanica being the most abundant (followed by Gephyrocapsa oceanica of a little lower abundance). The species indicate a truly shallow marine environment within recent sediment accumulation. Keywords: Surface sediments, littoral, calcareous nannofossils, Gulf of Guinea, Lagos, Nigeria, biostratigraphy, paleoenvironment, paleoecology, lithologic

Geochemistry of Ekenkpon and Nkporo shales, Calabar flank, SE Nigeria: implications for provenance, transportation history and depositional environment

The Cretaceous Nkporo and Ekenkpon Shales within the Calabar Flank were investigated using Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) and Atomic Emission Spectroscopy (ICP-AES) to deduce the provenance, transportation history and depositional environment. The results show the dominance of SiO2 (45.27 – 46.45%; 44.50 – 54.83%), Al2O3 (22.27 – 23.57%; 19.20 – 20.20%) and Fe2O3 (8.30 – 9.04%; 5.64 – 7.30%) constituting the bulk major oxides of Nkporo and Ekenkpon Shales respectively. The Index of Chemical Variation (ICV) ranges from 0.42 – 0.56 and 0.57– 0.68 for Nkporo and Ekenkpon Shales respectively, suggesting moderate and high degree of weathering. The enrichment of Sr (124 – 350ppm; Nkporo Shale and 176 – 856ppm; Ekenkpon Shale) compared to Post Australian Archean Shale (PAAS; 14.6ppm) could be attributed to feldspars in the source area, and is easily incorporated in the clay minerals of the shales. Also the values of Th and U which are 15.00 – 17.20ppm; 14.6- 23.2ppm and 3.5-4.2ppm; 1.9-6.2ppm respectively in Nkporo and Ekenkpon sediments show enrichment compared to PAAS of 14.6ppm and 3.1ppm for respective values of Th and U. The Eu anomaly of 0.04 to 0.07 suggests felsic source rocks for both formations while plots of TiO2 vs. Al2O3 indicate an intermediate to dominantly felsic granodiorite source for both shales, with little input from felsic volcanic provenance plus traces of quartzose sedimentary rocks. The mobility of Na, Ca, and K due to progressive weathering of the shales is evident in the bivariate plots of Na2O wt% vs. PIA, CaOwt% vs. PIA and K2O vs. PIA. Values of Th/U ratios range from 3.37 – 4.91(Nkporo) and 3.10 – 7.68 (Ekenkpon), indicating moderate to high weathering and reworking of sediments. It is envisaged that the area is associated with passive to active continental margin tectonics, where sediments were mainly sourced from felsic rocks of the adjoining terrain and deposited in oxic, continental to transitional marine environment based on Al2O3– K2O +CaO+MgO-Fe2O3+MgO; AKF plot.KEYWORDS: Cretaceous, Calabar Flank, provenance, transportation history, depositional environment

n-Alkane bound biomarker fingerprints from asphaltenes in the bitumens of Eastern Dahomey Basin, Nigeria: Source and genetic implications

The asphaltene fractions of the bitumens of Eastern Dahomey Basin in Nigeria, were analyzed by flash pyrolysis-gas chromatography (Py-GC) method in order to unravel its geochemical history and properties. The distributions of the initial biomarkers of the original oils from the pyrolysates are related to the assessment of organic matter source, paleo-redox conditions and source environment during deposition. Also, it effectively establishes the genetic relationship of the bitumens. The n-alkane distributions in the pyrolysates reveal nC$_9$-nC$_{32}$ n-alkanes, maximizing at nC$_{14}$, isoprenoids-pristane (Pr) and phytane (Ph), and some n-alkene peaks. High peaks of low to medium-weight nC$_9$-nC$_{20}$ n-alkanes and low peaks of nC$_{21}$+ n-alkanes characterize the distributions. These reveal that abundant algal organic matter with some terrigenous inputs contributed to the source rock of the bitumens. The high concentration of marine organic matter inputs to the source rock is further confirmed by the nC$_{17}$/nC$_{27}$ ratios which range from 5.39 to 19.82 and shows the predominance of nC$_{17}$ alkanes. The general unimodal n-alkane distributions in the bitumens indicate derivation from similar organic matter types showing that they are genetically related. The anoxic to suboxic environmental conditions that prevailed during the deposition of the sediments is revealed by the isoprenoids, Pr/Ph ratios (0.72–1.28). Pristane/nC$_{17}$ and Phytane/nC$_{18}$ range from 0.16 to 0.33 and 0.22 to 0.56, revealing that the bitumens were from predominantly marine organic matter (type II kerogen) preserved in a reducing environment with no evidence of biodegradation. However, the Ph/nC18 ratio and the cross plot of Pr+Ph/nC17+nC18 allow the classification of the bitumens into two subfamilies/groups (A and B). The bitumen samples have low wax content as indicated by the degree of waxiness ranging from 0.21 to 0.38 which confirms low terrigenous input. Based on the carbon preference index (CPI: 0.92 to 1.55) and odd-even predominance (OEP: 0.70 to 1.36), it is concluded that the bitumens are immature to marginally mature